Selective, Sequential, and “One-Pot” Depolymerization Strategies for Chemical Recycling of Commercial Plastics and Mixed Plastics

Abstract Chemical recycling of poly(L‐lactic acid) to the cyclic monomer L‐lactide is hampered by low selectivity and by epimerization and elimination reactions, impeding its use on a large scale. The high number of side reactions originates from the high ceiling temperature (T c) of L‐lactide, which necessitates high temperatures or multistep reactions to achieve recycling to L‐lactide. To circumvent this issue, we utilized the impact of solvent interactions on the monomer–polymer equilibrium to decrease the T c of L‐lactide. Analyzing the observed T c in different solvents in relation to their Hildebrand solubility parameter revealed a “like recycles like” relationship. The decreased T c, obtained by selecting solvents that interact strongly with the monomer (dimethyl formamide or the green solvent γ‐valerolactone), allowed chemical recycling of high‐molecular‐weight poly(L‐lactic acid) directly to L‐lactide, within 1–4 h at 140 °C, with >95 % conversion and 98–99 % selectivity. Recycled L‐lactide was isolated and repolymerized with high control over molecular weight and dispersity, closing the polymer loop.

“Like Recycles Like”: Selective Ring‐Closing Depolymerization of Poly(L‐Lactic Acid) to L‐Lactide

The societal importance of plastics contrasts with the carelessness with which they are disposed. Their superlative properties lead to economic and environmental efficiency, but the linearity of plastics puts the climate, human health, and global ecosystems at risk. Recycling is fundamental to transitioning this linear model into a more sustainable, circular economy. Among recycling technologies, chemical depolymerization offers a route to virgin quality recycled plastics, especially when valorizing complex waste streams poorly served by mechanical methods. However, chemical depolymerization exists in a complex and interlinked system of end-of-life fates, with the complementarity of each approach key to environmental, economic, and societal sustainability. This review explores the recent progress made into the depolymerization of five commercial polymers: poly(ethylene terephthalate), polycarbonates, polyamides, aliphatic polyesters, and polyurethanes. Attention is paid not only to the catalytic technologies used to enhance depolymerization efficiencies but also to the interrelationship with other recycling technologies and to the systemic constraints imposed by a global economy. Novel polymers, designed for chemical depolymerization, are also concisely reviewed in terms of their underlying chemistry and potential for integration with current plastic systems.

Depolymerization within a Circular Plastics System.

Polymer-based product usage rapidly increases globally, leading to severe ecological, social, environmental, health, and economic impacts. There has been an international push for solutions to sustainable production, consumption, and end-of-life for plastics to combat these issues. Moreover, additive manufacturing (AM) or 3D printing (3DP) processes provide rapid fabrication of functional parts in lesser time, lower lead times, and lower research and development costs. Due to these reasons, these processes are now being utilized in different industrial sectors, including aerospace, automotive, biomedical, sports, food, electronics, and construction. The circular economy concept integrated with AM processes can deliver a synergic impact and a new life to discarded polymeric parts with distributed recycling and manufacturing. With the development and adoption of the proper techniques, AM can be used widely to reuse polymer wastes to turn them into valuable products. It can lead to much higher levels of reuse at very low cost and for targeted applications. Therefore, in this study, a comprehensive literature review is performed to outline guidelines and a circular economy model for AM of recycled polymers based on the reviewed literature. This study delivers insight into; different commodity polymers and their uses, challenges in polymer recycling, polymer recycling approaches, and a circular economy model for AM of recycled polymers. 

Additive Manufacturing for Sustainability and Circular Economy: Needs, Challenges, and Opportunities for 3D Printing of Recycled Polymeric Waste

Poly(l-lactic acid) (PLLA) is a leading commercial polymer produced from biomass, showing useful properties for plastics and fiber applications; after use, it is compostable. One area for improvement is postconsumer waste PLLA chemical recycling to monomer (CRM), i.e., the formation of l-lactide (l-LA) from waste plastic. This process is currently feasible at high reaction temperatures and shows low catalytic activity accompanied, in some cases, by side reactions, including epimerization. Here, a commercial Sn(II) catalyst, applied with nonvolatile commercial alcohol, enables highly efficient CRM of PLLA to yield l-LA in excellent yield and purity (92% yield, >99% l-LA from theoretical max.). The depolymerization is performed using neat polymer films at low temperatures (160 °C) under a nitrogen flow or vacuum. The chemical recycling operates with outstanding activity, achieving turnover frequencies which are up to 3000× higher than previously excellent catalysts and applied at loadings up to 6000× lower than previously leading catalysts. The catalyst system achieves a TOF = 3000 h-1 at 0.01 mol % or 1:10,000 catalyst:PLLA loading. The depolymerization of waste PLLA plastic packaging (coffee cup lids) produces pure l-LA in excellent yield and selectivity. The new catalyst system (Sn + alcohol) can itself be recycled four times in different PLLA "batch degradations" and maintains its high catalytic productivity, activity, and selectivity.

Chemical Recycling of Commercial Poly(l-lactic acid) to l-Lactide Using a High-Performance Sn(II)/Alcohol Catalyst System

Plastics recycling has become an important issue based on the principle of sustainability and circular economy. Therefore, the development of a valuable chemical recycling method, to achieve the recycling process of end-of-life polylactide (PLA) waste without loss of material properties, meets unprecedented opportunities. In this regard, we propose a “DE–RE polymerization” strategy to achieve the recycling process from end-of-life PLA plastic waste to new virgin-quality PLA materials in a “polymer to polymer” mode. This approach improves the efficiency of PLA recycling and promotes the development of plastics reutilization. 

A “Polymer to Polymer” Chemical Recycling of PLA Plastics by the “DE–RE Polymerization” Strategy

Chemical upcycling of poly(lactide) plastic waste to lactate ester, lactide and new poly(lactide) under Mg-catalysis condition

Plastics are integral to numerous significant social advancements. Nonetheless, their contribution to environmental pollution and climate crises cannot be disregarded, as their negative impact on the environment increases with incremental production capacity and demand. Concerted global action is urgently required to promote the green recycle of plastics to prevent their accumulation in the environment and mitigate carbon emissions. This review aims to reveal the paths of green development for polyester plastics, incorporating the trends of the green revolution in mature commercial polyester plastics, newly emerging biodegradable polyester plastics, and future polyester plastics. A critical discussion was conducted on the current and potential future research areas from multiple perspectives, including raw materials, processes, and recycling, to propel us into a future marked by sustainability. 

Recycled Polymer: Green Roads for Polyester Plastics

Catalyst-free amino-alcoholysis depolymerization strategy: a facile and powerful tool for chemical recycling of poly(bisphenol A carbonate)

Isotactic polymers have emerged with unique and excellent properties in material sciences. Specific discrimination polymerization provides an ideal pathway to achieve highly isotactic polymers from their racemic monomers, which is of great significance and a challenge in polymeric chemistry. Although an enantioselective catalyst-mediated asymmetric kinetic resolution polymerization (AKRP) process makes it possible, a general and well-defined strategy for catalyst design is still rarely reported. Here, based on a novel dual-ligand strategy, a new type of chiral (BisSalen)Al complex with high enantioselectivity has been described, in which perfect AKRP of racemic phenethylglycolide (Pegl) is achieved for the first time. The more confined asymmetric microenvironment formed by a dual ligand is the key to improve the enantioselectivity of the original catalyst. To illustrate the generality of this strategy, a series of (BisSalen)Al complexes with homo- or heterodual ligands were designed for the AKRP of Pegl. 

Specific Discrimination Polymerization for Highly Isotactic Polyesters Synthesis.

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